The theory relating to acoustical imaging devices teaches that the point of optimum resolution of an acoustical-optical image tube is at or just below the fundamental resonant frequency of the acoustically active plate or face of the tube. The minimum distance between image points is directly proportional to the acoustical frequency. Thus it follows that for high resolution higher frequencies with shorter sound wave lengths must be used. Thus the resonant acoustic plate must also be thinner for use with the higher frequencies, and this produces some problems since the mechanical strength of the plate decreases as it gets thinner, thus setting a limit to the resolution and to the area of the acoustical plate. It has been found possible to support the plate at some points to increase its mechanical strength and prevent its bending toward the vacuum side by using mechanical supports. Another technique which has been tried to prevent mechanical failure of the front plate is to employ pressure equalizers in front of the plate. Still another method employs a spherically bent front plate with built-in extra strength against the deflection due to the pressure differential. With this latter technique, larger area plates can be used which means more resolution elements (picture elements) per tube face plate.
A somewhat better approach to this problem involves a tube face made of glass with many conduction metal pins extending through from inside to outside. Such tubes may be somewhat similar to cathode ray tubes in that they have an evacuated chamber for which the inside of the tube face forms a wall. The metal pin arrangement is vacuum-leakproof due to glass-to-metal seals at each pin. A typical arrangement of pins would include three per millimeter. An acoustically active piezoelectric plate is laid on the front plate of the tube or spaced therefrom and is thus outside of the tube. This design separates the acoustically active part and the vacuum-tight face plates functionally from each other. The acoustical piezoelectric plate does not carry an atmospheric pressure load; thus, it does not bend as it would if it were also the vacuum front window. Also, it does not have to go through the bake-out cycles which the tube itself goes through. Its size, thickness, composition, etc. are not determined or dictated by the vacuum practices followed in the construction of the tube. It is a somewhat independent item which is cemented or otherwise fastened to the front of the face of the finished tube.
It has been proposed to make tubes of this type with as many as 100 wires per square millimeter. This obviously would provide high resolution but at a cost in complexity of structure.